Is ${902315}$ divisible by $3$ ?
Answer: A number is divisible by $3$ if the sum of its digits is divisible by $3$ . [ Why? First, we can break the number up by place value: $ \begin{eqnarray} {902315}= &&{9}\cdot100000+ \\&&{0}\cdot10000+ \\&&{2}\cdot1000+ \\&&{3}\cdot100+ \\&&{1}\cdot10+ \\&&{5}\cdot1 \end{eqnarray} $ Next, we can rewrite each of the place values as $1$ plus a bunch of $9$ s: $ \begin{eqnarray} {902315}= &&{9}(99999+1)+ \\&&{0}(9999+1)+ \\&&{2}(999+1)+ \\&&{3}(99+1)+ \\&&{1}(9+1)+ \\&&{5} \end{eqnarray} $ Now if we distribute and rearrange, we get this: $ \begin{eqnarray} {902315}= &&\gray{9\cdot99999}+ \\&&\gray{0\cdot9999}+ \\&&\gray{2\cdot999}+ \\&&\gray{3\cdot99}+ \\&&\gray{1\cdot9}+ \\&& {9}+{0}+{2}+{3}+{1}+{5} \end{eqnarray} $ Any number consisting only of $9$ s is a multiple of $3$ , so the first five terms must all be multiples of $3$ That means that to figure out whether the original number is divisible by $3 $ , all we need to do is add up the digits and see if the sum is divisible by $3$ . In other words, ${902315}$ is divisible by $3$ if ${ 9}+{0}+{2}+{3}+{1}+{5}$ is divisible by $3$ Add the digits of ${902315}$ $ {9}+{0}+{2}+{3}+{1}+{5} = {20} $ If ${20}$ is divisible by $3$ , then ${902315}$ must also be divisible by $3$ ${20}$ is not divisible by $3$, therefore ${902315}$ must not be divisible by $3$.